The present invention relates to an electrically-heated chemical process reactor. More specifically, the invention relates to an electrically-heated screen heater located in a housing adapted for receiving and passing a flow of gases, wherein the gases flowing through the heater have a low residence time within the heater but are heated to a very high temperature while passing through the heater. The term “high temperature” in this case means temperatures in the range of from about 1600° F. to about 2500° F., and the term “low residence time” means a time on the order of about 10 milliseconds.
The heater described herein comprises an array of current-carrying wire screen element packets respectively arranged in series electrical connection and aligned in a channel for a gas-flow to pass through all of them, wherein the wire screen element packets may carry thousands of amperes of current at a relatively low voltage. The gas flow path is arranged to form an efficient, low pressure drop heat exchanger mechanism.
One advantage of the present invention over the prior art is that the ratio of heated surface area to gas volume is over ten times higher than that of prior art devices used for the same purposes, such as conventional gas-fired radiant furnaces and/or reactors. It has been demonstrated that the process gas temperatures approach the temperature of the heated wire screen element packets, thereby improving the heat exchanger effectiveness parameter.
A second advantage is the reduced structural requirement imposed on the heating elements. Prior art reactors suffer from temperature and pressure limitations because the tubes that heat the gas are also pressure-bearing components that are required for containing and transporting the gas. In the present invention, the heating elements are not required to contain or transport the gas, greatly reducing the structural requirement.
Hence, the present invention can run at higher temperatures than the prior art (by virtue of the reduced structural requirement) while delivering more of that temperature to the process stream (by virtue of the improved heat exchanger effectiveness).
Residence time and process temperature are the dominant parameters governing conversion and selectivity in a chemical conversion and/or cracking application. For example, in a well-known process for converting a mixture of ethane/steam to ethylene (mixture of approximately 3:1, ethane:steam), operating at a temperature of about 2000° F. and passing through a heater of approximately four to five feet in length, at Mach flow rates (the ratio of the local process gas flow velocity to the local speed of sound) of about 0.2 for a reasonable pressure drop through the heater, the typical prior art residence time exceeds 100 milliseconds (msec.). The invention described herein can produce the conversion required within a residence time of about 10 msecs.
The heater described herein is a very low-voltage, high-current device. Among the advantages of this invention are the fact that it can be operated in a direct current mode or in a single- or three-phase alternating current mode. The number, size and geometry of the screen element packets described herein can be adjusted or “tuned” to the specific needs of the process at hand, such as temperature and heat flux profile “tailoring”, as is frequently beneficial in various new and preexisting petrochemical processes. The length and cross-sectional flow area of the gas flow channel is selected to achieve the desired residence time for an optimum process gas temperature and pressure. The materials and configuration of the screens can be selected to maximize the screen operating temperature for various different conversion processes; the screen mesh wire diameters, screen size and shape, number of screens per packet, and number of packets provide a great deal of flexibility in configuring a process reactor to a desired heat flux profile or for a given gas processing or gas reaction requirement.
A novel feature of the invention is the construction of the heater array, providing a wedge clamping mechanism for clamping individual screen element packets to an electrical conductor block in a manner which provides a good electrical connection and relieves thermal stresses to the clamping mechanism, and to the entire array.
The present invention incorporates a novel heat exchange flow path to preheat gas flowing into the gas flow channel to a partial elevated temperature, to thereby improve the efficiency of heating the process gas to the desired operating temperature.
The high operating temperatures and associated materials inside the device produce significant temperature differences, both within the device and between the device and the components attached to the device, which lead to differential expansion and contraction. Therefore, the present invention incorporates an expansion joint/gas seal to ensure that the process gas is fully confined within the reactor under the extreme temperature of operation.